CN109901303B - Multi-mode optical fiber emergent light spot focusing method and system based on self-adaptive parallel coordinate algorithm - Google Patents
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Abstract
The invention provides a method and a system for rapidly focusing multi-mode fiber emergent light spots based on a self-adaptive parallel coordinate algorithm, aiming at solving the problem that batch focusing light spots cannot be rapidly formed at the emergent end of a multi-mode fiber in the existing multi-mode fiber emergent light spot focusing method. The method comprises online speckle acquisition and offline phase optimization; when each modulation sub-area of the space optical modulator is in a gating state, acquiring a non-interference speckle image at the exit end of the multimode fiber and an interference speckle image when the multi-mode fiber and the reference sub-area are in the gating state respectively, and scanning all M modulation sub-areas in a traversing manner to obtain 3(M-1) +1 speckle images. And for N preset focusing light spots, sequentially calculating the phase modulation state of each test mode corresponding to all focusing light spots by using the intensity information of corresponding focusing positions on 3(M-1) +1 speckle images to form a phase modulation mask, and loading the phase modulation mask on the spatial light modulator one by one to realize point-by-point focusing of N emergent light spots of the multimode fiber.
Description
Technical Field
The invention belongs to the technical field of novel optical imaging and light field regulation and control, and particularly relates to a method and a system for rapidly focusing emergent light spots of multimode optical fibers based on a self-adaptive parallel coordinate algorithm.
Background
The multimode fiber has a plurality of guided wave modes, can transmit information in parallel and realizes imaging in a region. In recent years, multimode fiber imaging methods have become a focus of research. However, when the incident light of the multimode fiber is transmitted in different guided wave modes, the energy exchange between the modes is caused by mode dispersion and mode coupling, the time and space information of the incident light field is disturbed, and signal crosstalk is generated, so that the output of the multimode fiber is a series of speckles, and the application of the multimode fiber in the transmission of digital signals and image signals is hindered.
At present, the multimode fiber imaging method is mainly divided into a light field transmission matrix method and a focused light spot scanning method, wherein the light field transmission matrix method needs to measure the light field transmission matrix in advance, the calculated amount is very large, the measurement accuracy of the light field transmission matrix is influenced by the bending of the fiber, and the flexibility and the stability of an experimental system need to be improved. The focusing light spot scanning method does not need to measure a light field transmission matrix, has a simple light path and good system stability, but forms a focusing light spot at the tail end of the multimode fiber, uses the focusing light spot as a sampling light spot, scans an object to be measured for imaging, the number of the light spots usually reaches hundreds or even thousands, and in order to enable the scanning speed to be faster, the focusing speed of the light spot must be faster.
The existing multi-mode fiber emergent light spot focusing method comprises a focusing method based on digital phase conjugation and a method for forming a focused light spot based on wave front modulation, wherein the focusing method based on digital phase conjugation measures a scattered light field of a point light source after being transmitted by a multi-mode fiber, and can form focusing at the position of an original point light source according to a light path reversible principle after complex conjugation operation is carried out on the scattered light field. The method does not need to measure a light field transmission matrix, an iterative process is not needed, but the position of the optical element has large influence on the conjugate phase, and an experimental system is also greatly influenced by environmental noise. The method for forming the focusing light spots based on the wavefront modulation is to modulate the wavefront of incident light through a dynamic diffraction optical element, so that the focusing light spots can be formed at the emergent end of the optical fiber after the light is transmitted through the multimode optical fiber, a light field transmission matrix does not need to be measured, the optical path is simple, and the system stability is good. However, the multi-mode fiber emergent light spot focusing method needs multiple iterations, repeatedly visits external equipment, has large calculation amount, and takes longer time to form batch focused light spots.
Disclosure of Invention
The invention provides a method and a system for rapidly focusing multi-mode fiber emergent light spots based on a self-adaptive parallel coordinate algorithm, aiming at solving the technical problem that batch focusing light spots cannot be rapidly and effectively formed at the emergent end of a multi-mode fiber in the existing multi-mode fiber emergent light spot focusing method.
The technical scheme of the invention is as follows:
a method for rapidly focusing emergent light spots of multimode optical fibers based on a self-adaptive parallel coordinate algorithm is characterized by comprising the following steps:
step 1) on-line speckle Collection
1.1) adopting a spatial light modulator to perform phase modulation on incident light coupled into a multimode fiber;
dividing the spatial light modulator into M modulation subregions, taking one modulation subregion as a reference mode, and taking the other M-1 modulation subregions as test modes;
Smmodulating the size of a subregion for a preset spatial light modulator; sallThe size of the image surface of the spatial light modulator which can be coupled into the multimode optical fiber; the spatial light modulator is a device capable of modulating the phase of incident light;
1.2) selecting a reference modal region of the spatial light modulator and gating;
1.3) collecting an interference-free speckle image of the exit end of the multimode fiber;
1.4) turning off the reference mode area of the spatial light modulator;
1.5) selecting and gating a first test mode region of the spatial light modulator;
1.6) collecting an interference-free speckle image of the exit end of the multimode fiber;
1.7) gating a reference mode region of the spatial light modulator; (the reference mode region at this time is a reference mode region without overlapping pi/2 phase);
1.8) collecting a first interference speckle image of an exit end of the multimode fiber;
1.9) modulating the phase of a reference mode of the spatial light modulator to enable the reference mode to be superposed with the phase of pi/2;
1.10) collecting a second interference speckle image of the multi-mode fiber exit end;
1.11) judging whether all M-1 test modes are scanned completely, if not, closing the reference mode, gating the next test mode, and returning to the step 1.6); if yes, ending the collection, obtaining 3(M-1) +1 speckle images in total, and entering the step 2).
Step 2), performing off-line phase optimization;
2.1) setting N focusing light spots at different positions; the upper value limit of N is the ratio of the size of the emergent speckle area of the multimode fiber to the size of a focusing spot area, and the lower value limit of N is 2; the position of the focusing light spot is set in the speckle image range of the multimode fiber exit end;
2.2) selecting non-interference speckle images corresponding to the first test mode and the reference mode respectively, and two-time interference speckle images of the two modes;
2.3) calculating the optimized phase modulation state of all N focusing light spots in the current test mode;
2.4) judging whether the optimal phase modulation states of all N focusing light spots under all M-1 test modes are calculated, if not, selecting non-interference speckle images respectively corresponding to the next test mode and the reference mode and two-time interference speckle images of the two modes, and turning to the step 2.3); if so, stopping the algorithm, and obtaining N phase modulation masks required by the spatial light modulator corresponding to the N focusing light spots in total;
and 2.5) loading N phase modulation masks corresponding to the N focusing light spots onto the spatial light modulator one by one, and modulating light beams output by the laser to realize point-by-point focusing of the N emergent light spots of the multimode fiber.
Further, the size of the modulation sub-region in step 1.1) is P × Q spatial light modulator pixels, where P and Q are both positive integers.
Further, in step 1.1) above, the reference mode is selected in the center of the M modulation subregions.
Further, the step 2.3) is specifically as follows:
recording the total light intensity of the corresponding position of the reference mode non-interference speckle image as IrefTest mode interference-free speckle image corresponding bitThe total light intensity is recorded as ItestAnd the total light intensity of the corresponding position of the first interference speckle image of the reference mode and the test mode is recorded as I1And the total light intensity of the corresponding position of the second interference speckle image of the reference mode and the test mode is recorded as I2By the formula
And
the phase difference delta between the reference mode and the test mode can be calculated, and the value of delta is between (0,2 pi).
The invention also provides a multimode fiber emergent light spot rapid focusing system of the multimode fiber emergent light spot rapid focusing method, which is characterized in that: the device comprises a laser, a collimation and beam expansion module, a spatial light modulator, a 4f system, a focusing objective, an imaging objective and a CCD camera which are sequentially arranged along a light path;
the collimation and beam expansion module is used for collimating and expanding beams of light beams emitted by the laser and then irradiating the light beams to the spatial light modulator;
the spatial light modulator is a device which can have a phase modulation effect on incident light;
the 4f system is used for gating only the 0 th order diffraction light reflected by the spatial light modulator;
the focusing objective lens is used for focusing the 0-order diffraction light onto the front end face of the multimode optical fiber;
the imaging objective lens is used for imaging light spots on the rear end face of the multimode optical fiber to the CCD camera and receiving the light spots by the CCD camera;
the system for rapidly focusing the emergent light spot of the multimode optical fiber based on the self-adaptive parallel coordinate algorithm further comprises a processor and a memory, wherein a computer program is stored in the memory, and the steps of the method are realized when the computer program is run by the processor.
Further, the collimation and beam expansion module comprises a first lens and a second lens which are sequentially arranged along the emergent light path of the laser.
Further, the 4f system includes a first convex lens, an aperture stop and a second convex lens sequentially arranged along the optical path, the aperture stop is arranged on the focal plane of the first convex lens and the focal plane of the second convex lens, and the first convex lens is used for receiving the reflected light beam of the spatial light modulator.
Further, the spatial light modulator is an optical phase shifter.
Compared with the prior art, the invention has the following beneficial effects:
1. in the online speckle collection process, all the modulation subregions of the spatial light modulator are traversed once, and the phase modulation mask which meets the requirement that all target positions form focusing light spots can be obtained through offline phase optimization, so that the phase modulation mask which has the advantages of minimum number of times of online accessing the modulation subregions of the spatial light modulator, no iteration, small calculation amount and capability of parallelly obtaining point-by-point focusing of a plurality of light spots is realized. Therefore, the focusing method of the invention is simple, convenient and effective and has high modulation speed.
2. The invention can form batch focusing light spots only by once scanning, and can be applied to the related application fields of rapid light field regulation, biomedical imaging and the like.
3. When the focusing method is realized, the optical path structure is simple, the optical path stability is good, and the calculation is simple.
Drawings
FIG. 1 is a schematic diagram illustrating a system for implementing rapid focusing of an emergent light spot of a multimode fiber based on an adaptive parallel coordinate algorithm according to an embodiment.
FIG. 2 is a flowchart illustrating a method for implementing point-by-point focusing of multi-mode fiber emergent spots according to an embodiment.
FIG. 3 is a schematic diagram of an online speckle collection process in a method for realizing multi-mode fiber emergent light spot point-to-point focusing according to an embodiment.
FIG. 4 is a diagram showing the focusing light spot effect at 30 different positions obtained by focusing the emergent light spots of the multimode optical fiber point by point once according to the invention.
Reference numerals:
1. the device comprises a laser, 2, a first lens, 3, a second lens, 4, a spatial light modulator, 5, a first convex lens, 6, an aperture diaphragm, 7, a second convex lens, 8, a focusing objective lens, 9, a multimode fiber, 10, an imaging objective lens, 11, a CCD camera, 12, a computer, 13-a collimation and beam expansion module and 14-4f system.
Detailed Description
The invention is further described below with reference to the accompanying drawings and examples.
As shown in fig. 1, the system for rapidly focusing an emergent light spot of a multimode optical fiber based on an adaptive parallel coordinate algorithm provided in this embodiment includes a laser 1, a first lens 2, a second lens 3, a spatial light modulator 4, a first convex lens 5, an aperture stop 6, a second convex lens 7, and a focusing objective 8, which are sequentially disposed along an optical path; the rear end of the focusing objective 8 is connected with an imaging objective 10 through a multimode optical fiber 9; a CCD camera 11 is arranged on an emergent light path of the imaging objective lens 10; the CCD camera 11 is connected to a computer 12.
The first lens 2 and the second lens 3 constitute a collimation and beam expansion module, and are used for making a collimated beam emitted by the laser 1 enter the spatial light modulator 4 after being collimated and expanded.
The aperture diaphragm 6 is arranged on the focal plane of the first convex lens 5 and the second convex lens 7, the first convex lens 5, the aperture diaphragm 6 and the second convex lens 7 form a 4f system, only 0-order diffraction light reflected by the spatial light modulator 4 is gated for phase modulation, and other stray light is filtered; the gated 0 th order diffracted light is focused on the front end face of the multimode fiber 9 through the focusing objective lens 8, and the light spot on the rear end face of the multimode fiber 9 is incident to the CCD camera 11 through the imaging objective lens 10 to be received.
Based on the embodiment of the system shown in fig. 1, the method for realizing the point-by-point focusing of the emergent light spots of the multimode optical fiber specifically comprises the following steps:
step 1) on-line speckle Collection
1.1) the spatial light modulator 4 is divided into M modulation sub-regions, each modulation sub-region corresponding to H spatial light modulator pixels.
The number M of the modulation subregions is related to the image plane size of the spatial light modulator which can be coupled into the multimode fiber and the size of each modulation subregion of the spatial light modulator which is preset. The number of pixels H of the spatial light modulator corresponding to each modulation subarea is related to the size of each modulation subarea of the preset spatial light modulator and the size of a single pixel of the spatial light modulator.
The size of the image surface of the spatial light modulator capable of being coupled into the multimode optical fiber is SallThe size of each modulation subarea of the preset spatial light modulator is SmThe size of a single pixel of the spatial light modulator is S0Then the number of modulation sub-regions on the spatial light modulator is
I.e. M is SallAnd SmRounded down the ratio of (a). The number of pixels of the spatial light modulator corresponding to each modulation subregion
If S is the same as the image plane size of the spatial light modulator coupled into the multimode fibermIf the modulation range is too small, the influence of environment vibration on the system is too large, the signal-to-noise ratio is low, and the number of the modulation subregions is too large, so that the calculated amount of data processing is increased; smToo large will result in too few modulation subregions and thus an inability to precisely control the optical field of the multimode fiber, resulting in poor spot focus quality.
In this embodiment, M is 1024, and H is 10 is 100.
Each modulation sub-region corresponds to 10 × 10 — 100 spatial light modulator pixels, which are relatively optimized parameters determined through a large number of experiments, and other values (e.g., 5 × 5, 20 × 20) can also achieve focusing.
1.2) selecting one of the modulation subregions as a reference mode (the position of the reference mode should be as close to the center as possible to ensure that the modulation subregions can be completely coupled into the multimode fiber), and using the other modulation subregions as test modes to sequentially generate a spatial light modulator modulation map for non-interference speckle image acquisition and interference speckle image acquisition as shown in the left side of fig. 3, wherein 3(M-1) +1 is 3070 sheets in total; the spatial light modulator modulation graph is a phase modulation mask graph (composed of 0-255) which can be loaded on the spatial light modulator, and can be generated by MATLAB software, wherein the gray value of the modulation sub-region is 255, which represents that the phase of the sub-region is 2 pi, and the gray value is 0, which represents that the phase of the sub-region is 0.
1.3) loading all phase modulation mask images (3070) on the spatial light modulator 4 in sequence, using the CCD camera 11 to perform traversal scanning once, and acquiring all non-interference speckle images and interference speckle images;
2) offline phase optimization
2.1) setting N focusing spots at different positions (in this embodiment, N is 5 × 6 is 30, and 5 rows and 6 columns of focusing spots at different positions) and setting the size of each focusing spot to be 5 × 5 CCD pixels; theoretically, the upper limit of the number of the focused light spots is the ratio of the emergent speckle area size of the multimode fiber to the area size of one focused light spot, but in particular practice, the time of off-line calculation should be considered. Presetting several rows and several lines has no special requirement, and does not need to be continuous positions as long as the position of the focusing light spot is set in the speckle image range of the multimode fiber exit end.
In this step, the number of CCD pixels corresponding to each focused spot is determined by calibrating the pixel equivalent of the CCD before the experiment. Firstly, the exit end face of the multimode fiber is clearly imaged by using a CCD camera to obtain the number of pixels of the exit end face of the multimode fiber occupying the CCD camera, and then the size of an object plane corresponding to each pixel of the CCD, namely the pixel equivalent weight, can be obtained. Then, dividing the size of the ideal focusing light spot by the pixel equivalent to obtain the CCD pixel number corresponding to each focusing light spot. The size of the ideal focused spot is defined as the full width at half maximum of the fiber exit airy disk, determined by the numerical aperture of the multimode fiber.
2.2) calculate N phase modulation masks required by M-1(═ 1023) test modes for the spatial light modulators corresponding to the N focused spots (each focused spot phase modulation mask is composed of the phase modulation states of (M-1) test modes and the modulation states of 1 reference mode with a constant phase of 0):
2.2.1 selecting a non-interference speckle image corresponding to the first test mode and the reference mode respectively and two-time interference speckle images of the two modes;
2.2.2) focus on the total intensity of the 5 × 5 pixels corresponding to the first focused spotRecording the total light intensity of the corresponding position of the reference mode non-interference speckle image as IrefAnd the total light intensity of the corresponding position of the test mode non-interference speckle image is recorded as ItestAnd the total light intensity of the corresponding position of the first interference speckle image in the test mode is recorded as I1And the total light intensity of the corresponding position of the second interference speckle image in the test mode is recorded as I2By the formula
And
the phase difference delta between the reference light and the test light can be calculated, and the value of delta is between (0,2 pi).
2.2.3) similarly and sequentially acquiring optimized phase modulation states (0-2 pi) corresponding to the 30 focusing light spot images under the current test mode by using the method in the step 2.2.2);
2.2.4) selecting the non-interference speckle images corresponding to the next test mode and the reference mode respectively and the two-time interference speckle images of the two modes;
2.2.5) repeating steps 2.2.2) -2.2.4) until an optimized phase modulation state of 30 focused light spots in M-1 (1023) test modes is obtained, at which time 30 phase modulation masks corresponding to the 30 focused light spots respectively are obtained (each phase modulation mask is composed of phase modulation states of M-1 (1023) test modes and a modulation state of 1 reference mode with a constant phase of 0).
2.3) sequentially loading the phase modulation masks corresponding to the 30 focusing light spots obtained in the step 2.2.5) on the spatial light modulator 4, thereby realizing the point-by-point focusing of the 30 emergent light spots of the multimode fiber.
2.4) shooting the focusing light spots at different positions of the 30 emergent ends of the multimode optical fiber obtained in the step 2.3) by using a CCD camera 11, wherein an effect graph is shown in FIG. 4. As can be seen from FIG. 4, the invention can form a batch of focusing spots with better quality in a shorter time by only one-time scanning, and provides scanning sampling spots for the scanning imaging of the focusing spots of the multimode fiber.
Claims (7)
1. A multi-mode fiber emergent light spot focusing method based on a self-adaptive parallel coordinate algorithm is characterized by comprising the following steps:
step 1), online speckle collection;
1.1) adopting a spatial light modulator to perform phase modulation on incident light coupled into a multimode fiber;
dividing the spatial light modulator into M modulation subregions, taking one modulation subregion as a reference mode, and taking the other M-1 modulation subregions as test modes;
wherein SmModulating the size of a subregion for a preset spatial light modulator; sallThe size of the image surface of the spatial light modulator which can be coupled into the multimode optical fiber;
1.2) selecting a reference modal region of the spatial light modulator and gating;
1.3) collecting an interference-free speckle image of the exit end of the multimode fiber;
1.4) turning off the reference mode area of the spatial light modulator;
1.5) gating a first test mode region of the spatial light modulator;
1.6) collecting an interference-free speckle image of the exit end of the multimode fiber;
1.7) gating a reference mode region of the spatial light modulator;
1.8) collecting a first interference speckle image of an exit end of the multimode fiber;
1.9) modulating the phase of a reference mode of the spatial light modulator to enable the reference mode to be superposed with the phase of pi/2;
1.10) collecting a second interference speckle image of the multi-mode fiber exit end;
1.11) judging whether all M-1 test modes are scanned completely, if not, closing the reference mode, gating the next test mode, and returning to the step 1.6); if yes, ending the collection to obtain 3(M-1) +1 speckle images, and entering the step 2);
step 2), performing off-line phase optimization;
2.1) setting N focusing light spots at different positions; the upper value limit of N is the ratio of the size of the emergent speckle area of the multimode fiber to the size of a focusing spot area, and the lower value limit of N is 2; the position of the focusing light spot is set in the speckle image range of the multimode fiber emergent end;
2.2) selecting a non-interference speckle image corresponding to the first test mode and the reference mode respectively and two-time interference speckle images of the two modes;
2.3) calculating the optimized phase modulation state of all N focusing light spots in the current test mode;
recording the total light intensity of the corresponding position of the reference mode non-interference speckle image as IrefAnd the total light intensity of the corresponding position of the test mode non-interference speckle image is recorded as ItestAnd the total light intensity of the corresponding position of the first interference speckle image of the reference mode and the test mode is recorded as I1And the total light intensity of the corresponding position of the second interference speckle image of the reference mode and the test mode is recorded as I2By the formula
And
solving the phase difference delta between the reference light and the test light, wherein the delta value is between (0,2 pi);
2.4) judging whether the optimal phase modulation states of all N focusing light spots under all M-1 test modes are calculated, if not, selecting non-interference speckle images respectively corresponding to the next test mode and the reference mode and two-time interference speckle images of the two modes, and turning to the step 2.3); if so, stopping the algorithm, and obtaining N phase modulation masks required by the spatial light modulator corresponding to the N focusing light spots in total;
and 2.5) loading N phase modulation masks corresponding to the N focusing light spots onto the spatial light modulator one by one, and modulating light beams output by the laser to realize point-by-point focusing of the N emergent light spots of the multimode fiber.
2. The method for focusing the emergent light spot of the multimode optical fiber based on the adaptive parallel coordinate algorithm according to claim 1, wherein the method comprises the following steps: in step 1.1), the size of the modulation sub-region is P × Q spatial light modulator pixels, and P and Q are both positive integers.
3. The method for focusing the emergent light spot of the multimode optical fiber based on the adaptive parallel coordinate algorithm according to claim 1, wherein the method comprises the following steps: in step 1.1), the reference mode is located at the center of the M modulation subregions.
4. A multimode fiber emergent light spot fast focusing system based on a self-adaptive parallel coordinate algorithm is characterized in that: the device comprises a laser (1), a collimation and beam expansion module (13), a spatial light modulator (4), a 4f system (14), a focusing objective lens (8), an imaging objective lens (10) and a CCD camera (11) which are sequentially arranged along a light path;
the collimation and beam expansion module (13) is used for collimating and expanding beams emitted by the laser (1) and then irradiating the beams to the spatial light modulator (4);
the spatial light modulator (4) is used for carrying out phase modulation on incident light;
the 4f system (14) is used for gating only the 0 th order diffraction light reflected by the spatial light modulator (4);
a focusing objective lens (8) for focusing the 0 th order diffracted light onto a front end face of a multimode optical fiber (9);
the imaging objective lens (10) is used for imaging light spots on the rear end face of the multimode fiber (9) to the CCD camera (11) and receiving the light spots by the CCD camera (11);
the system for fast focusing the emergent light spot of the multimode optical fiber based on the adaptive parallel coordinate algorithm further comprises a processor (12) and a storage, wherein a computer program is stored in the storage, and the computer program realizes the steps of the method according to any one of claims 1 to 3 when the computer program is executed by the processor (12).
5. The system according to claim 4, wherein the system comprises: the collimation and beam expansion module (13) comprises a first lens (2) and a second lens (3) which are sequentially arranged along the emergent light path of the laser (1).
6. The system according to claim 4, wherein the system comprises: the 4f system (14) comprises a first convex lens (5), an aperture diaphragm (6) and a second convex lens (7) which are sequentially arranged along a light path, wherein the aperture diaphragm (6) is arranged on a focal plane of the first convex lens (5) and the second convex lens (7), and the first convex lens (5) is used for receiving a reflected light beam of the spatial light modulator (4).
7. The system according to claim 4, wherein the system comprises: the spatial light modulator (4) is an optical phase shifter.
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